Here is the full list of all 69 Deep Space Waifu: FLAT JUSTICE achievements. It takes around 2-3 hours to unlock all of the achievements on Windows.
DATE all girls
Find the PERFECT FLATNESS
Finish with the M.I.L.F.
Complete ONE HANDED MILF
Defeat Dickus final form
100% M.I.L.F.
Infiltrate in the secret TENTACLE organization
Destroy the Sneak Mothership
complete 100% the spy
Unlock M.I.L.F.
Defeat DP Miner
100% SERGEANT CHIKARA
Unlock the SECRET SPY
Get the super scret spy business card
Defeat Dickus third form
100% LIME CODE
Be spotted by the enemy
Unlock SERGEANT CHIKARA
Defeat the Thrasher Super Mecha Warrior
Reach 30 STARS
Complete NORMAL ROCKY-CHAN
Unlock LIME CODE
Defeat DP Mothership
100% METAL ROCKY CHAN
Unlock NORMAL ROCKY-CHAN
Get the F.L.A.T. Multipass badge
Defeat Dickus second form
100% PUCKY K-9 UNIT
Unlock PUCKY K-9 UNIT
Get promoted to the F.L.A.T. SPECIAL FORCES
Get a smelling Pantie evidence
Complete SAYRU-CHAN
Reach 18 STARS
Defeat the Fake Wadding
100% SHARP-EYE YUMIKO
Unlock METAL ROCKY-CHAN
Defeat DP Defender
Reach 10 STARS
100% NAO NEKOYAMA
Get the SHIELD power up
Explode a mine!
Unlock SAYRU-CHAN
Unlock SHARP-EYE YUMIKO
Get a special Phone Number
Defeat Dickus first form
100% JULIA SUZUHARA
Make a 5 STARS stage
Get the MACHINE GUN power up
Get the RAY power up
Take a photo of a girl
Kick Justice in the balls
100% MIKA MOKOTO
Use the JUSTICE LASER special attack
Get the LASER power up
Get the ROCKET LAUNCHER power up
Torn apart your first piece of cloth
Go to your first date
Begin history mode
Complete ONE HAND JULIA
Complete ONE HAND NAO
Complete ONE HAND MIKA
Complete an ONE HANDED SALE
Complete ONE HANDED SHARP-EYE
Complete ONE HAND METAL ROCKY CHAN
Complete ONE HAND NORMAL ROCKY CHAN
Complete ONE HAND PUCKY K-9 UNIT
Complete ONE HAND SERGEANT CHIKARA
Complete ONE HAND LIME CODE
complete ONE HANDED SPY